I think that its importance has been overestimated. The radiative forcing
effect of methane relative to carbon dioxide—in other words, its global
warming potential (GWP)—has been estimated by the Intergovernmental Panel
on Climate Change to be 25 times as great. This value is obtained by integrating
the direct and indirect radiative forcing effects of methane over 100 years.

However, this calculation assumes that all methane has the same effect
regardless of its source. This assumption is not correct. Fossil-fuel-related
methane returns carbon to the atmosphere on a geological timescale, so
it can be considered to add new carbon to the carbon cycle. By contrast, methane
emitted from the biosphere—from rice paddies, wetlands, waste dumps,
biomass burning and enteric fermentation in herbivores—represents a
redistribution of carbon already in the cycle.

Only the direct radiative forcing effect of biospheric methane should be
included with the direct and indirect effects of fossil fuel-related methane in
calculating its GWP. As the biosphere contributes some 70 to 80 per cent of the
methane now entering the atmosphere, the IPCC’s value of 25 for the GWP of
methane significantly overestimates its contribution to the enhanced greenhouse
effect.